1. Field of the Invention
The invention relates to a method of feeding a dampening water to a plate cylinder in an offset printing press, and an apparatus therefor.
2. Description of Related Art
In an offset printing press, there has been commercially available an apparatus for feeding a dampening water to a plate cylinder, as disclosed in Japanese Laid-Open Patent Publication No. 6863 of 1990. The apparatus may be a spray-type which includes a plurality of spray nozzles opposed to a fountain roller. The fountain roller is disposed in contact with a dampening roller. A supply of dampening water is connected to the spray nozzles through a plurality of solenoid operated valves to spray the dampening water onto the fountain roller. The fountain roller cooperates with the dampening roller to feed the dampening water to the plate cylinder. The printing press includes an equipment for feeding an ink to the plate cylinder, the dampening water preventing the ink from adhering to the plate, so that the ink will adhere to the plate selectively and partially for printing.
In the case, it is required that the dampening water corresponds in feeding amount to the ink. As to the feeding amount of ink, it is concerned with the speed of the printing press. On the other hand, the printing press is started at the beginning of printing to be accelerated to and operated at a predetermined speed. The printing press is then decelerated from the predetermined speed to be stopped at the end of printing. The ink increases and decreases in feeding amount in accordance with the increase and decrease in speed of the printing press when the printing press is accelerated and decelerated. In this connection, it is required to conveniently control the feeding amount of the dampening water so that the dampening water increases and decreases in feeding amount in accordance with the increase and decrease in speed of the printing press when the printing press is accelerated and decelerated, to correspond in feeding amount to the ink.
Under the circumstances, an attempt is made to establish a characteristic curve C0 which is predetermined to indicate the relation of the feeding amount of dampening water to the speed of printing press, as shown in FIG. 3. At the beginning of printing, the printing press is accelerated to a predetermined speed between points S1 and S2. The apparatus conveniently controls the feeding amount of the dampening water so that the dampening water increases in feeding amount in accordance with the increase in speed of the printing press along the characteristic curve C0, to correspond in feeding amount to the ink. The printing press is then operated at the predetermined speed for printing. The point S1 represents the speed in rotation of the plate cylinder of about 500 rpm to print about 120,000 news papers per hour. The point S2 represents the speed in rotation of the plate cylinder of about 420 rpm to print about 100,000 news papers per hour. The news paper has a length of 456 cm. At the end of printing, the printing press is decelerated from the predetermined speed. The apparatus conveniently controls the feeding amount of the dampening water so that the dampening water decreases in feeding amount in accordance with the decrease in speed of the printing press along the same curve C0, to correspond in feeding amount to the ink. The printing press is accelerated and decelerated gradually at the beginning and end of the printing for adjustment to print clearly and not to dirty the product with excess ink.
In the case, the printing press prints 2,000 to 3,000 news papers at a time when accelerated or decelerated. In addition, the printing press is accelerated and decelerated frequently whenever the printing press is stopped for a trouble and whenever certain country version plates are exchanged for other country version ones. Accordingly, it prints a large number of news paper in all when accelerated and decelerated.
However, the printing press can not have a high response in feeding amount of the ink, in particular with respect to the decrease in speed of the printing press when the printing press is decelerated. Accordingly, there may be a tendency to feed relatively more ink when the printing press is decelerated, resulting in lack of the dampening water. This causes dirt of news paper to waste many materials. On the contrary, it may be required according to the kind of ink and news paper material to feed relatively more dampening water, in particular when the printing press is accelerated, to rapidly remove the dirt of paper and minimize the waste of material.
It is therefore an object of the invention to provide a new and improved method of feeding a dampening water to a plate cylinder in an offset printing press, and an apparatus therefor, to overcome the above problems.
Another object of the invention is to minimize the waste of material due to lack of dampening water when the printing press is accelerated and decelerated.